A conventional radar system employing a frequency modulated continuous wave (FMCW) method generates a triangular transmitted signal whose frequency varies linearly in a fixed frequency band as shown in FIG. 9, emits the transmitted signal into a space as a radar wave, and receives a scattered wave of the transmitted signal reflected from a target and returned.
Then, it mixes the transmitted signal with the received signal to generate a beat signal, and computes a complex amplitude (beat spectrum) corresponding to the frequency of the beat signal (beat frequency) by carrying out Fourier transform of the beat signal.
In addition, when the conventional FMCW radar system computes the beat frequency of the beat signal, it identifies the beat frequency in each sweep interval of an up-chirp in which the frequency of the transmitted signal increases and of a down-chirp in which the frequency decreases, and computes range R to a target and relative velocity V as shown in the following Expressions (1) to (4) from the beat frequency fup at the up-chirp and the beat frequency fdown at the down-chirp.
                              f          up                =                              -                                          2                ⁢                B                            cT                                +                                    2              ⁢                              Vf                0                                      c                                              (        1        )                                          f          down                =                                            2              ⁢              B                        cT                    -                                    2              ⁢                              Vf                0                                      c                                              (        2        )                                R        =                              -                          cT                              4                ⁢                B                                              ⁢                      (                                          f                up                            +                              f                down                                      )                                              (        3        )                                V        =                              -                          cT                              4                ⁢                                                                  ⁢                                  f                  0                                                              ⁢                      (                                          f                up                            -                              f                down                                      )                                              (        4        )            
Here, B is the frequency deviation width (sweep bandwidth) of the transmitted signal, f0 is the center frequency of the transmitted signal, T is time (sweep time) taken to carry out modulation of one period.
In addition, V represents the relative velocity between the vehicle and a target, in which the direction of approach is made +frequency, and C is the speed of light.
Incidentally, it is assumed that since the modulation time T is short, the range R to a target and the relative velocity V are invariant during the up-chirp or down-chirp.
Therefore, it can compute the range R to a target and the relative velocity V, and a technique for distinguishing between a moving target and a stationary object using the FMCW radar system is disclosed in the following Patent Document 1.
To distinguish between the moving target and the stationary object, the radar system disclosed in the Patent Document 1 utilizes the fact that when the vehicle is running at velocity −Va, a stationary object approaches at the relative velocity Va so that the relative velocity V in Expression (4) becomes Va.
More specifically, it shifts at least one of the peaks of the beat spectra at the up-chirp and at the down-chirp by a prescribed frequency, computes the degree of spectral match between the beat spectrum at the up-chirp and the beat spectrum at the down-chirp whose peaks correspond to each other, and recognizes that the object associated with the peaks is a stationary object if the degree of the spectral match is high.